In order to realize a shortest possible stopping distance of automobiles in emergency brake situations, it is necessary to excessively raise the brake pressure with respect to a pedal force initiated by the driver. Since studies have yielded the result that, in emergency brake situations, normal drivers often cannot or only with a delay induce the required pre-pressure, so-called “brake assistant systems” were developed which raise the brake pressure automatically above the level pre-determined by the driver.
In principle, there are three known systems. In a first one, the brake pressure is raised by means of an automatic control of the brake booster. In the second one, the active pressure raise occurs through suitable control of the ABS/ESP hydraulics by means of the electric return pump. Further, there are other known brake assistant systems which all will be further developed. Just for example, it is referred to mechanical or electrical/mechanical brake assistants.
A method to shorten the stopping distance in critical driving situations has been disclosed in the German Patent DE 40 28 290 C1. In the method disclosed in this document, the excess of a first threshold value by the actuation speed of the brake pedal initiated by the driver is the criterion for the release of an automatic brake event, where, immediately after the release of the automatic brake event, such a brake pressure is automatically built up which corresponds to the value of the brake pressure at optimum vehicle deceleration. In order to ensure that the excessively raised brake pressure is reduced when the necessity of an automatic brake event is eliminated, it is verified, according to the teaching of this document, whether the actuation force of the brake pedal is smaller than a pre-set threshold value, i.e. whether the vehicle driver wants to reduce the power of the brake event and thus only a brake event with a lower brake force is necessary.
A mode of operation is provided which provides for the transition from a full pressure build-up of the actual brake assistant to a conventional brake behavior in order to avoid an abrupt termination of the support provided by the brake assistant which, immediately upon termination of the brake assistant, could have the result that a relatively low tandem master cylinder pressure coincides with a relatively high locking pressure.
Therefore, the objective of the present invention consists in avoiding the disadvantages of the prior art and in indicating a method of operating a brake assistant which avoids an abrupt termination of the brake support and which, at the same time, is particularly safe and user-friendly.
This objective is achieved in a method of the kind mentioned above by means of the present invention. Here, excess elevation is not understood as being the, regarding its absolute value, higher wheel cylinder pressure raised by the brake assistant with respect to the tandem master cylinder pressure, but it is rather the relative amount of this excess elevation with respect to the pressure in the tandem master cylinder.
A special advantage of the invention consists in that a once-initiated transition to conventional braking behavior is not being maintained for a longer time period. Otherwise, this would, e.g. while going downhill, result in an undesired and unsafe state of operation.
Preferably, the excess elevation is dependent on the driving situation and/or the input of a vehicle driver my means of the brake pedal. Thus, the brake force support can be tuned to the driving conditions in the best possible way. Also, in this case, a harmonic correlation can be ensured between the driver's directive and the pressure raise.
Preferably, the rate at which the excess elevation is reduced increases with the time duration and/or the intensity of a reduction of pedal force induced by the vehicle driver. A reduction of pedal force indicates a driver's intention that a brake operation is not necessary or not necessary as forceful any more. The input control by the driver can be used in an advantageous way to design the transition of the brake assistant function to the conventional brake operation.
A particularly simple and cost-effective realization of the invention results from using a counting device to recognize whether and/or by what amount the driver reduces the pedal force.
In order to implement the invention, it is preferred if the momentary value of the wheel brake pressure results substantially from a multiplication of a current value of a time-dependent excess elevation function and the current value of the tandem master cylinder pressure.
And the excess elevation function, as a function of time, is monotonously descending.
Preferably, the excess elevation function descends in time segments in which the master cylinder pressure descends. Further, the excess elevation function is constant in time segments in which the tandem master cylinder pressure increases. Thus, every diminution of the induced brake force effects a reduction of the excess elevation, and every other input via the brake pedal affects the wheel brake pressure but not the excess elevation. In this way, the brake assistant support can be diminished unnoticeably for the driver.
According to a preferred enhancement of the invention, a momentary value of the excess elevation function depends on the previous course of the tandem master cylinder pressure. The consideration of the history of the tandem master cylinder pressure is particularly preferred for the estimate of the driving situation and of the driver's intention. Advantageously, the excess elevation function includes a pre-set maximum value. In this way, implausible excess elevations of the wheel brake pressure can be avoided.
The brake assistant system preferably changes over from the third mode of operation to the first mode of operation when the excess elevation function substantially occupies the value “1”. In this case, the driver himself affords the required locking pressure and does not need any further support.